This invention relates to a process for forming one or more substantially pure layers of an element in a material. More particularly, this invention relates to the formation of one or more substantially pure layers of an element in another element, compound, or solid solution by ion implantation.
Implantation of small amounts of ions of a particular element into a substrate such as crystalline silicon is well known in the construction of integrated circuit structures. For example, elements such as boron, phosphorus, or arsenic are implanted into a silicon substrate in small amounts to alter the electrical properties of the silicon. It is known to implant oxygen atoms into a silicon substrate in large concentrations to form silicon oxide layers by the combination or reaction of the implanted oxygen ions with the silicon in the substrate.
For example, Baerg et al. U.S. Pat. No. 4,700,454, teaches the implantation of a silicon substrate with oxygen ions to form an insulating layer and states that other ions which will react with silicon to form an insulating layer may be used instead of oxygen, such as nitrogen, to form a silicon nitride-like layer. Short et al., U.S. Pat. No. 4,749,660, teaches implanting sufficient oxygen ions into a silicon substrate to form a maximum concentration of two oxygen atoms per silicon atom to form a buried SiO.sub.2 layer in the silicon substrate. Griffith, U.S. Pat. No. 4,786,608, also teaching the formation of a silicon oxide buried layer in a silicon substrate by oxygen ion implantation.
Implantations of other elements into an elemental substrate without reaction of the substrate atoms and the implanted ions to form a compound such as the above-described silicon oxide compound are also known. For example, Myers and Smugeresky, in an article entitled "Phase Equilibria and Diffusion in the Be-Al-Fe System Using High Energy Ion Beams", published in Metallurgical Transactions, Vol. 7A, at pp. 795-802 in 1976, describe the implantation of single crystal beryllium with 30-50 keV aluminum in dosages of 2-6.times.10.sup.16 aluminum atoms/cm.sup.2 (i.e., peak aluminum concentrations &lt;10 atomic %) followed by annealing at temperatures of 600.degree. C. and 800.degree. C. The authors reported observation of aluminum precipitates using transmission electron microscopy (TEM) after annealing.
Buene et al., in an article entitled "Metastable Alloys of Beryllium Prepared by Ion Implantation", published in Metallurgical Transactions, Vol. 15A, at pp. 1787-1804 in 1984, reported the implantation of beryllium with zinc at an energy of 185 keV with a 1.6.times.10.sup.17 zinc atoms/cm.sup.2. Annealing at 600.degree. C. for 10 minutes resulted in an increase in the peak zinc concentration of from 12 to about 17 atomic %.
However, others have not reported the formation of a substantially pure layer of an implanted element formed in another element, compound, or solid solution as a result of ion implantation. This is not surprising since sputtering of both the implanted substrate as well as atoms of the implanted element during implantation would appear to limit the maximum concentration of the implanted element in the substrate obtainable using ion implantation. Furthermore, diffusion of the implanted element away from the highest concentration regions during both the implant and the subsequent annealing step would also appear to limit the maximum concentration of the implanted element in the substrate obtainable by implantation. Finally, precipitation of intermediate phases containing elements of the substrate and the implanted element would be expected to interfere with the formation of the elemental layer.